318iS M20 swap questions

[Digitalwave]

I've got a spare M20B25 that I am swapping into a '91 318iS. I will carry over the G260 and wiring harness from the M20. My questions are:

1) Is the only difference between the brake boosters on a 318iS and 325iS the location of the vacuum line nipple? It's on the driver's side of the booster on the 325, and on the passenger's side of the booster on the 318.

2) Are the fuse boxes the same? I noticed the relays look a bit different on the 2 cars, should I swap all of the 325 relays over?

[AndrewBird]

Brake booster should be fine as it is matched to the brakes, not the engine.

All the relays in the fuse box are for things like headlights are unrelated to the engine. Main/fuel pump/etc relays are on the engine harness.

[nando]

careful with pin 20 of the C101. On early cars it's sometimes a ground, but on late cars it's the trigger for the ABS relay. if your engine/harness is from a 90 or 91, it should be fine, but if it's an 89 or earlier, you'll need to deal with the harness so that it triggers the relay properly and doesn't short 12v to ground.

[richardb]

The prices on the 91 318is cars have really gone up in the last few years. I would suggest repairing or replacing the stock engine instead of swapping an M20. People are seeking out those 318is cars!

[Digitalwave]

careful with pin 20 of the C101. On early cars it's sometimes a ground, but on late cars it's the trigger for the ABS relay. if your engine/harness is from a 90 or 91, it should be fine, but if it's an 89 or earlier, you'll need to deal with the harness so that it triggers the relay properly and doesn't short 12v to ground.

Thanks, I had not heard about that before. Donor car is an '89 and the 318 is a build date of 04/90. Any suggestions on how to test for that?

The prices on the 91 318is cars have really gone up in the last few years. I would suggest repairing or replacing the stock engine instead of swapping an M20. People are seeking out those 318is cars!

Thanks for your $0.02. I just bought the car from it's original owner and it's being stripped down to a track car. Slick top that's never been in an accident = perfect to put a cage in.

[Digitalwave]

Chris,

I checked both harnesses today. On the 325i (12/88 build), pin 20 is empty on both the harness and body side.

On the 318iS (04/90 build), pin 20 is red/yellow on both sides (for the ABS trigger).

Does that mean I just need to add a switched 12v pin on to the engine harness to trigger the body side ABS on the 318iS chassis?

Everything else should be the same right?

[nando]

yep! just emulate the abs wiring for the late 325i/318is.

[Digitalwave]

Just to update this thread:

We ran a new wire from the switched 12v pole of the main relay to pin 20 of the M20 engine harness. This worked perfectly to power the late model ABS system, which is powered by pin 20 of the C101 connector. The ABS worked perfectly. Nothing else needed to be changed for the M20 swap. If your engine harness is also a 1990 or 1991, you won't need to make this change. If the harness is 1989 or older where the ABS was wired differently, you'll have to add the 12v switched to pin 20. Check the ETM for both the donor car and the chassis to verify the wiring for ABS.